Add solution for 'Linked List Cycle' LeetCode problem.

hw14/141_LinkedListCycle.php

@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * Definition for a singly-linked list.
+ * class ListNode {
+ *     public $val = 0;
+ *     public $next = null;
+ *     function __construct($val) { $this->val = $val; }
+ * }
+ */
+
+class Solution
+{
+    /**
+     * @param ListNode $head
+     * @return Boolean
+     */
+    public function hasCycle(ListNode $head): bool
+    {
+        if ($head === null) {
+            return false;
+        }
+
+        $node = $head;
+        $nextNode = $head->next;
+
+        while ($node !== $nextNode) {
+            if ($node === null || $nextNode === null) {
+                return false;
+            }
+
+            $node = $node->next;
+            $nextNode = $nextNode->next->next;
+        }
+
+        return true;
+    }
+}